Refactor AnimationBase to manage m_isAccelerated explicitly.

There are three major refactoring points.
1. m_isAccelerated: When accelerated animations start, it becomes true. When
accelerated animations end, it becomes false.
2. Don't go accelerated animation path if m_isAccelerated is false.
3. Change isAccelerated() semantics. It is possible that m_isAccelerated is true
and m_object->isComposited() is false, because of style change.

BUG=297641

Review URL: https://codereview.chromium.org/24601002

git-svn-id: svn://svn.chromium.org/blink/trunk@158610 bbb929c8-8fbe-4397-9dbb-9b2b20218538